Help ? Redox titrations

5)Calculate the volume of 0.0200 mol dm-3 potassium manganate (VII) which just reacts with 0.142 g of iron (II) sulphate,
in acid solution.

6) Ammonium iron (II) sulphate crystals have the following formula: (NH4)2SO4.FeSO4.nH2O. In an experiment to find n,
8.492 g of the salt were dissolved and made up to 250 cm3
solution with distilled water and dilute sulphuric acid. A 25.0
cm3
portion of the solution was titrated against 0.0150 mol dm-3 KMnO4, 22.5 cm3
being required. Calculate n.

7) A piece of iron wire weighs 2.225 g. It is dissolved in acid, which oxidises it to Fe2+, and made up to 250 cm3. A 25
cm3
sample required 31.0 cm3
of a 0.0185 mol dm-3 solution of potassium dichromate. Calculate the percentage of iron
in the wire.

8) A 25.0 cm3
aliquot of a solution containing Fe2+ and Fe3+ ions was acidified and titrated against 0.0200 M potassium
manganate (VII) solution, requiring 15.0 cm3
. Zn reduces Fe3+ to Fe2+ and a second aliquot was reduced by zinc and
after filtering off the excess zinc, was titrated with the same potassium manganate solution, requiring 19.0 cm3
.
Calculate the concentrations of the Fe2+ and Fe3+ in the solution.
I don't want to do your homework.

Have you tried to do it, yet?

Where have you got up to?
Original post by Pigster
I don't want to do your homework.

Have you tried to do it, yet?

Where have you got up to?

its now 7 years later. I am some random kid who has the same homework. Instead of complaining you should've just done it to help out others like me
Original post by BOBBLE.D.69
5)Calculate the volume of 0.0200 mol dm-3 potassium manganate (VII) which just reacts with 0.142 g of iron (II) sulphate,
in acid solution.

6) Ammonium iron (II) sulphate crystals have the following formula: (NH4)2SO4.FeSO4.nH2O. In an experiment to find n,
8.492 g of the salt were dissolved and made up to 250 cm3
solution with distilled water and dilute sulphuric acid. A 25.0
cm3
portion of the solution was titrated against 0.0150 mol dm-3 KMnO4, 22.5 cm3
being required. Calculate n.

7) A piece of iron wire weighs 2.225 g. It is dissolved in acid, which oxidises it to Fe2+, and made up to 250 cm3. A 25
cm3
sample required 31.0 cm3
of a 0.0185 mol dm-3 solution of potassium dichromate. Calculate the percentage of iron
in the wire.

8) A 25.0 cm3
aliquot of a solution containing Fe2+ and Fe3+ ions was acidified and titrated against 0.0200 M potassium
manganate (VII) solution, requiring 15.0 cm3
. Zn reduces Fe3+ to Fe2+ and a second aliquot was reduced by zinc and
after filtering off the excess zinc, was titrated with the same potassium manganate solution, requiring 19.0 cm3
.
Calculate the concentrations of the Fe2+ and Fe3+ in the solution.

QUESTION 8
1) again first write the redox equation:

MnO4- + 8H+ + 5Fe2+ -------> Mn2+ + 5Fe3+ + 4H2O

2) Work out Moles of what you know (Manganate again)

moles MnO4- = conc * V(cm^3) / 1000
= 0.0150* 22.5/1000
=3.38*10^-4 mol of MnO4-

3.) next use ratios to find moles of Fe2+ in 25cm^3

MnO4- : Fe2+
1:5

therefore moles of Fe2+ = moles of MnO4- * 5
= (3.38*10^-4) * 5
= 1.69*10^-3 mol of Fe2+ in 25cm^3

4.) work out moles of Fe2+ in the original 250cm^3 solution

..... 25 * 10 = 250

therefore (1.69*10^-3) * 10 = 0.0169mol of Fe2+ in the 250cm^3 solution

5.) Moles of Fe2+ are equal to the moles of the whole salt , as for every unit of (NH4)2SO4.FeSO4.nH2O there is one unit of FeSO4

6.) we can therefore work out the Mr of the salt

Mr = grams/moles
= 8.492 / (0/0169)
= 502(3sf)

7.) Next work out the Mr of (NH4)2SO4.FeSO4 using your periodic table

so for N (2*14)
for H (8*1)
for S (2*32.1)
for O (8 * 16)
for Fe(1*55.8)
TOTAL = 284

So minus 284 form the total Mr of the salt to gwt the Mr of the water of crystallisation

502-284 = 218

Mr of 1 molecule of H2O is (1*2)+(1*16)= 18

therefore number of water molecule (n)
...... = 218/18
= 12.11 (2dp)

So value of n is 12

you may get slightly different answer depending on how many decimal places you use but you will still get something along the lines of 12.11 or 12.16 but the answers still 12 anyways